Transmission systems with homodyne or heterodyne detection (hereinafter referred to, as a whole, as coherent communication systems) are already well known in radio-frequency communications, and can be used in optical communications, especially at long wavelengths such as those comprised within the so-called second or third transmission windows (1.3-1.6 .mu.m). In fact, at these wavelengths direct-detection system performance is limited by detector sensitivity (sensitivity is here defined as the minimum input power necessary to yield a predetermined error rate). Germanium detectors or detectors based on compounds of Periodic Table Group III and Group V elements are intrinsically more noisy than silicon detectors which can be utilized in the first window.
By contrast, coherent communication systems allow a sensitivity close to the limits due to quantum noise in photoelectric conversion. By conversion of the optical carrier into radio-frequencies, the selectivity of electronic filters can be used in optical transmissions, thus allowing a more complete exploitation of the available fiber band in case of FDM (frequency division multiplex) communications.
Various optical-fiber coherent communication systems are already known, which use amplitude, frequency or phase or differential phase modulation. A comparative analysis of the performances of these systems with one another and with direct-detection systems has been made e.g. in the papers "Computation of Bit-Error-Rate of Various Heterodyne and Coherent-Type Optical Communication Schemes" by T. Okoshi, K. Emura, K. Kikuchi, R.Th. Kersten, Journal of Optical Communications, Vol. 2 (1981), N.3, pages 89-96, and "Coherent Fiberoptic Communications" by D.W. Smith, Laser Focus/Electro-Optics. November 1985, pages 92-106. These analyses show that the best performance as to sensitivity is obtained by phase-modulation systems, followed by frequency and amplitude modulation systems. All these systems, as already mentioned, have better performance than direct detection systems.
Yet, the coherent systems suggested till now require, as light sources, lasers with an extremely narrow line to limit phase noise or detection. The higher the sensitivity required, the more stringent the linewidth constraints. More particularly, for frequency or amplitude modulation systems the linewidth cannot exceed 20% of the bit rate used for transmission, while for phase modulation systems the linewidth required is of the order of some thousandths of the bit rate.
At the bit rates nowadays obtainable without considerable difficulties, these requirements cannot be met by commercially available semiconductor-lasers. Sources are described in the literature such as the so-called distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers, with linewidth characteristics which render them usable for amplitude or frequency modulation transmissions. Such sources are not yet commercially available.
Sources with the linewidths necessary to phase modulation transmissions, at bit rates of practical interest, are obtained by coupling a semiconductor laser with an external cavity. However, these are laboratory solutions, since such sources are too complicated, of low reliability and too difficult to handle for field use.